(1) Field of the Invention
This invention relates to a simplified method of forming phase shifting type masks and more particularly to a method of forming phase shifting type masks using a single resist layer with two exposure doses.
(2) Description of the Related Art
Phase shifting masks provide a significant improvement in process margin for the fabrication of integrated circuit wafers. Light projected through the masks expose a layer of photoresist formed on the integrated circuit wafer. The phase shifting masks have phase shifting material to shift the phase of the light at the edge of pattern features, usually by 180.degree.. This shift in phase causes beneficial interference at the edge of pattern features and improves feature definition.
U.S. Pat. No. 4,935,334 to Boettiger et al. describes a method of forming a photoresist pattern from a mask. The photoresist pattern can have different wall profiles than the mask pattern used to form the photoresist pattern.
U.S. Pat. No. 5,780,188 to Rolson describes a system for exposing a layer of resist on a semiconductor wafer. The layer of resist is exposed in multiple stages. During each stage the target can be exposed through unequal stepping distances.
U.S. Pat. No. 5,783,337 to Tzu et al. describes a method for forming attenuating phase shifting masks. A transparent mask substrate has a layer of phase shifting material formed thereon and a layer of chrome attenuating phase shifting material formed on the layer of phase shifting material. A layer of resist is formed on the layer of chrome. The layer of resist is exposed using two different exposure levels to pattern the layer of phase shifting material and the layer of chrome attenuating phase shifting material.
U.S. Pat. No. 5,759,724 to Rolson describes a method of forming multi-phase phase shifting masks.
U.S. Pat. No. 5,413,884 to Koch et al. describes a method of using a direct-write electron beam process to form rectangular grating teeth in a mask substrate. The mask is then used as a phase mask to transfer the pattern to an optoelectronic device.
U.S. Pat. No. 5,582,939 to Pierrat describes a method of forming defect-free phase shift masks.
A paper entitled "Primary Process in E-Beam and Laser Lithographies for Phase-Shift Mask Manufacturing", by Takahashi et al., SPIE Vol. 1674, Optical/Laser Microlithography V (1992), pages 216-229 discusses two lithographic processes for phase-shift mask manufacturing.
A paper entitled "Primary Process in E-Beam and Laser Lithographies for Phase-Shift Mask Manufacturing. II", by Kurihara et al., SPIE Vol. 1809 12th Annual BACUS Symposium (1992), pages 50-61 discusses two lithographic processes for phase-shift mask manufacturing.